1. Field of the Invention
The present invention relates to a rivet gun, especially to a mandrel-pulling distance sensing assembly of an electric rivet gun.
2. Description of the Prior Art(s)
A rivet is a kind of fastener that can hold at least two working pieces together and includes a mandrel and a rivet body mounted on an end of the mandrel. A rivet gun holds and pulls the mandrel, causing the rivet body to deform and hold the working pieces between a flange of the rivet body and a deformed portion of the rivet body.
A conventional electric rivet gun has a motor and a snapping device that is used for snapping the mandrel. The motor has a rotating shaft. A rotational motion of the rotating shaft is converted into a linear motion via a gear set, such that the snapping device and the mandrel are pulled to move linearly. Therefore, by setting a number of rotations of the rotating shaft of the motor, a linear distance that the snapping device and the mandrel are pulled can be controlled. However, rotational speeds of the rotating shaft of the motor differ with regard to input voltages and the different rotational speeds directly influence a time difference which is occurred when the rotating shaft is switched to rotate reversely. Moreover, after long-term use of the gear set and the motor, teeth of the gear set may be worn down or impurities may accumulate between the teeth of the gear set, causing inaccuracy of positioning the snapping device at specific positions by setting the number of rotation of the rotating shaft of the motor.
In another conventional electric rivet gun, the linear distance that the snapping device and the mandrel are pulled is set by two photo interrupters. One of the photo interrupters is disposed at a start position at which the snapping device has not been pulled. The other photo interrupter is disposed at an end position at which the mandrel of the rivet breaks. Operation of the motor is controlled by the photo interrupters, and accordingly a moving range of the snapping device can be controlled. However, due to ways of mounting the photo interrupters and impurities that accumulate in the electric rivet gun, positions of the photo interrupters and projection angles of light of the photo interrupters may offset. Accordingly, accuracy of the start position and the end position are influenced. Moreover, the light from each of the photo interrupters forms an illumination range. The moving range of the snapping device may differ with regard to the illumination range.
Therefore, no matter positions of the snapping device is controlled by the number of rotations of the rotating shaft of the motor or by the photo interrupters, the accuracy of positioning the snapping device at specific positions is low. The linear distance that the snapping device and the mandrel are pulled cannot be accurately controlled.
To overcome the shortcomings, the present invention provides a mandrel-pulling distance sensing assembly of an electric rivet gun to mitigate or obviate the aforementioned problems.